This invention relates to the gasification of carbonaceous solids and is particularly concerned with a catatytic gasification process utilizing a catalyst containing potassium sulfate.
It has long been recognized that certain alkali metal compounds can be employed to catalyze the gasification of carbonaceous materials such as coal and other carbonaceous solids. Studies have shown that potassium carbonate, sodium carbonate, cesium carbonate and lithium carbonate will substantially accelerate the rate at which steam, hydrogen, carbon dioxide, oxygen and the like react with bituminous coal, subbituminous coal, lignite, petroleum coke, organic waste materials and similar carbonaceous solids to form methane, carbon monoxide, hydrogen, carbon dioxide and other gaseous products. Other alkali metal salts such as potassium sulfate, however, have a low catalytic activity when compared to that of the corresponding carbonate and will only accelerate the gasification reactions at a small fraction of the rate obtainable with the alkali metal carbonates. It has been found that of the alkali metal carbonates, cesium carbonate is the most effective gasification catalyst, followed by potassium carbonate, sodium carbonate and lithium carbonate, in that order. Because of the relatively high cost of cesium carbonate and the low effectiveness of lithium carbonate, most of the experimental work in this area which has been carried out in the past has been directed toward the use of potassium and sodium carbonate. The catalytic activity of sodium carbonate, however, is substantially lower than that of potassium carbonate, therefore attention has been focused in the past on the use of potassium carbonate as a gasification catalyst.
In addition to utilizing an individual alkali metal salt as a catalyst for the gasification of a carbonaceous material, it has been proposed to utilize mixtures of alkali metal salts and mixtures comprising an alkali metal hydroxide and an alkaline earth metal hydroxide or carbonate. It has also been proposed to use a mixture of sodium and calcium cations that are ion exchanged onto lower ranking coals such as lignites. When mixtures of catalytic compounds possessing different activities are used to promote the gasification of a carbonaceous feed material, it is expected that the mixture will accelerate the gasification reactions less than if an equivalent amount of the more active compound is used alone and more than if an equivalent amount of the less active compound is used alone. In a recent publication concerning the use of catalysts in coal gasification it was concluded that there is a substantial need for additional research in general areas relating to the use of catalysts in coal gasification. Specifically, it was suggested that a study of catalyst combinations would be a promising area for future research.
In gasification processes utilizing alkali metal-containing catalysts, the cost of the catalyst is a significant factor in determining the overall cost of the product gas. Potassium carbonate is relatively expensive, costing approximately $12.48 per pound mole of potassium. Thus, when potassium carbonate is utilized as a catalyst it is essential that the potassium constituents in the spent solids produced during gasification of the carbonaceous feed material be recovered and reused in the process in order to maintain catalyst cost at a reasonable level. When these potassium constituents are removed from the spent solids exiting the gasifier by water leaching, it has been found that only a portion of the potassium carbonate is recovered and that substantial quantities of makeup alkali metal compounds are therefore required. This adds appreciably to the cost of the gasification operation. In order to decrease the amount of alkali metal makeup compounds necessary, it has been suggested to further treat the char from the gasifier to recover water-insoluble alkali metal constituents by more sophisticated and expensive recovery techniques.
Potassium sulfate, which costs about $2.12 per pound mole of potassium is substantially cheaper than potassium carbonate but has been found to possess only a fraction of the catalytic activity exhibited by potassium carbonate. It would be highly desirable if relatively inexpensive potassium sulfate could be effectively used as a gasification catalyst thereby substantially decreasing the initial investment required in the catalyst and obviating the need for expensive secondary recovery techniques to decrease the amount of makeup catalytically active compounds that would otherwise be required to maintain the catalyst inventory at the required level.